1. Field of the Invention
The present invention relates to a radio base station which communicates with multiple mobile stations through radio traffic channels, and more particularly to a multi-carrier TDMA radio base station and a method of traffic channel assignment in which multiple carrier frequencies and multiple time slots which are formed at each carrier frequency on a time-division basis are combined selectively thereby to establish multiple radio traffic channels.
2. Description of the Prior Art
Among the digital cordless telephone systems, some systems adopt the multi-carrier TDMA system for the access scheme of radio channels. The multi-carrier TDMA is capable of providing a total of N-by-M channels by using, for each base station, multiple (N) carrier frequencies each having the formation of multiple (M) TDMA time slots.
As stated in the RCR STD-28 (A standard of Research and Development Center for Radio System), each mobile station of a digital cordless telephone system is rendered the assignment a traffic channel (information channel) by the base station through the control channel at the commencement of communication or at the handover which occurs when the mobile station moves from one cell area to another cell area, and it carries out communication by using the assigned traffic channel.
Each base station of the multi-carrier TDMA telephone system responds to a request of traffic channel assignment from a mobile station to select a channel that meets the specified communication quality, e.g., CIR (carrier to interference power ratio) above a certain threshold, from among the multiple (N-by-M) channels, and assigns the selected channel to the mobile station.
There are two known schemes of channel selection. The FA (first available) scheme selects at random a combination of a carrier frequency and a time slot position which determines the starting channel for the quality check, measures the communication quality of the channel while shifting the combination sequentially, and assigns the first found channel that meets the specified quality to the mobile station. The worst CIR scheme measures the quality of all N-by-M channels, and assigns the channel of the worst quality out of the channels which meet the specified quality to the mobile station. The worst CIR scheme is advantageous in the efficiency of use of frequencies, while it suffers from a longer time expended for channel assignment.
With the intention of overcoming the drawback of the worst CIR scheme, there is proposed a channel assignment scheme in Japanese Patent Unexamined Publication No. Hei 6-209283, for example, in which one time slot is selected out of multiple time slots, the communication quality is measured for channels of N in number having the selected time slot and different carrier frequencies, and the carrier of the lowest quality among the carriers that meet the specified quality is selected for assignment to the mobile station. This modified worst CIR scheme is designed to assign a channel to a mobile station based on the FA scheme by varying the time slot position if no carrier that meets the specified communication quality is found for the first-selected time slot.
In regard to the radio communication system of multi-carrier TDMA, a high-speed data communication service which needs to assign multiple traffic channels to one mobile station is under study. In this case, however, due to the limited communication functions installed on mobile stations, restriction is imposed on a set of multiple traffic channels assigned to one mobile station, e.g., channels may have a same carrier frequency and different time slots, or channels may have different carrier frequencies and discontinuous time slots.
In this respect, in the conventional channel assignment schemes which assign traffic channels at random to requesting mobile stations, unused channels are left cumulatively in the form of irregular channel setup which represents the combinations of carrier frequencies and time slots, resulting highly possibly in a call blocking caused by the failure of channel assignment for a request of simultaneous multi-channel assignment from a mobile station due to the absence of remaining channels that meet the required condition of the mobile station.
Moreover, the conventional channel assignment schemes are designed to commence the channel selecting operation upon receiving a channel assignment request, resulting adversely in a longer time expended for channel assignment for a multi-channel assignment request, with the above-mentioned special restriction being imposed.
An object of the present invention is to provide a method of traffic channel assignment and a base station which are capable of responding quickly to a request of multi-channel assignment from a mobile station.
Another object of the present invention is to provide a method of traffic channel assignment and a base station which are capable of reducing the occurrence of call blocking for a request of multi-channel assignment from a mobile station.
In order to achieve the above objectives, the base station of multi-carrier TDMA radio communication system based on this invention comprises a management table memory for registering at least one group of candidate channels for multi-channel communication selected from among multiple traffic channels defined in terms of the carrier frequency and time slot number, and a channel assigning unit which, in response to the reception of a channel assignment request from a mobile station of multi-channel type that uses multiple channels, assigns traffic channels to the mobile station by making reference to the management table memory.
More specifically, the candidate channel group for multi-channel communication registered in the management table memory comprises multiple candidate channels having a certain mutual relation in regard to the channel arrangement in terms of the carrier frequency and time slot number, and the channel assigning unit selects channels that are suited for the requesting mobile station from among the multiple candidate channels included in the candidate channel group.
A feature of the present invention is that the channel assignment unit includes a means of measuring the communication quality of a candidate channel, which has been selected with reference to the management table memory, in cooperation with the requesting mobile station, and a means which, if candidate channels of the specified communication quality to be assigned to the mobile station are absent in the management table memory, searches for free channels of the specified communication quality to be assigned to the mobile station from among other traffic channels which are not registered in the management table memory.
According to a preferred embodiment of this invention, the base station of a multi-carrier TDMA radio communication system comprises a first management table for registering at least one candidate channel for single-channel communication selected from among multiple traffic channels defined in terms of the carrier frequency and time slot number, a second management table for registering at least one group of candidate channels for multi-channel communication selected from among the multiple traffic channels, and a channel assignment unit which, in response to the reception of a channel assignment request from a mobile station, assigns a traffic channel to the mobile station by making reference to one of the first and second management tables selectively depending on the type of the requesting mobile station.
Reference is made to the first management table in case the requesting mobile station is of single-channel type, or to the second management table in case it is of multi-channel type.
The first and second management tables have their contents updated at the commencement or termination of communication by a mobile station, or updated when the channel is switched, or updated periodically. The updating process of the second management table for multi-channel communication takes precedence over the first table for single-channel communication, and multiple groups of free channels selected in the specified order from among multiple channels formed by multi-carrier TDMA are registered in it.
As to whether a channel is available or not can be discriminated based on the received RF signal level. According to one embodiment of this invention, the candidate channel which has been checked to be a free channel based on the received RF signal level is examined as to whether it is an effective candidate channel by implementation of communication with at least one mobile station.
In case a mobile station of multi-channel type which was assigned a first channel for multi-channel communication has initiated the calling procedure without requesting a second channel, the second channel which is left in the second management table becomes idle. In this case, according to one embodiment of this invention, the base station reduces the transmission power of the first channel intentionally in order to let the mobile station request the channel switching, so that it assigns a traffic channel resulting from referencing to the first management table in response to the channel switching request. The first channel is freed by the channel switching, and it is registered as a candidate channel for multi-channel communication back in the second management table at the next management table updating process.